Chemical-mechanical polishing (CMP) is used in semiconductor fabrication processes for obtaining full planarization of a semiconductor wafer. The method involves removing material (e.g., a sacrificial layer of surface material) from the wafer, (typically silicon dioxide (SiO2)) using mechanical contact and chemical erosion from, e.g., a moving polishing pad saturated with slurry. Polishing flattens out height differences, since areas of high topography (hills) are removed faster than areas of low topography (valleys). FIG. 1A shows a top view of a CMP machine 100, and FIG. 1B shows a side section view of the CMP machine 100 taken through line AA. The CMP machine 100 is fed wafers to be polished. Typically, the CMP machine 100 picks up a wafer 105 with an arm 101 and places it onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is often textured, to aid the polishing process. The polishing pad 102 rotates on a platen 104 or turn table located beneath the polishing pad 102 at a predetermined speed. The wafer 105 is held in place on the polishing pad 102 by the arm 101. The lower surface of the wafer 105 rests against the polishing pad 102. The upper surface of the wafer 105 is against the lower surface of the wafer carrier 106 of arm 101. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 against the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense arm 107 extending across the radius of the polishing pad 102. The slurry dispense arm 107 dispenses a flow of slurry onto the polishing pad 102.
Over time the polishing pad loses its roughness and elasticity, and thus, its ability to maintain desired removal rates (polishing rates). It is known that the material removal rate provided by a given polishing pad decreases exponentially with time in the manner shown in FIG. 2. Further the decreased removal rate requires ever-increasing conditioning parameters, e.g., down force and/or conditioning angular velocity and/or conditioning time, in order to restore the desired removal rate of material from the wafer. As a consequence, the polishing pad must be conditioned (e.g., using a conditioning disk 108), between polishing cycles. The conditioning disk is held in place on the polishing pad by arm 109. As the polishing pad rotates, the conditioning disk 108 also rotates. Doing so roughens the surface of the pad and restores, at least temporarily, its original material removal rate. Furthermore, excessive pad conditioning shortens pad life.
A problem with conventional conditioning methods is that they may over-condition, e.g., wear out prematurely, the polishing pad. Each time a pad is replaced, one to several wafers must be polished thereon and the results measured, to ensure that the tool will yield the required polishing. This translates into processing delays and lost tool efficiency.
In an attempt to extend the life of the pad, one may selectively condition portions a polishing pad, or vary the down force of the conditioning element (e.g., conditioning disk 108) along the surface of the CMP pad, based upon the distribution of waste matter across the planarizing surface. Other methods of extending pad life include varying the conditioning recipe across the surface of the polishing pad in response to polishing pad non-uniformities. However, these reported CMP processes are typically more concerned with improving the CMP process, e.g., improving within water non-uniformity, than in extending pad life.
Methods and devices that would extend pad life and therefore reduce the frequency of pad replacement offer significant cost savings to the wafer fabrication process.